Powder coatings are an alternative to traditional liquid based coatings and paints. Liquid based systems are fairly easy to tint and color to produce a desired color. However, this has not been generally true of powder-based systems due to the manufacturing process as well as the application process for powders.
Powder coatings are generally manufactured in a multi-step process. Various ingredients, which may include resins, curing agents, pigments, additives, and fillers, are dry-blended to form a premix. This premix is then fed into an extruder, which uses a combination of heat, pressure, and shear to melt and thoroughly mix the ingredients. As the ingredients mix together in melted form during the extrusion process, various colors of pigments can be mixed together to produce a uniform color. The extrudate is cooled and then ground into a powder. Depending on the desired coating end use, the grinding conditions are typically adjusted to achieve a powder median particle size of about 25-150 μm.
The final powder may then be applied to an article by various means including the use of fluid beds and spray applicators. Most commonly, an electrostatic spraying process is used, wherein the particles are electrostatically charged and sprayed onto an article that has been grounded so that the powder particles are attracted to and cling to the article. After coating, the article is heated. This heating step causes the powder particles to melt and flow together to coat the article.
Optionally, continued or additional heating may be used to cure the coating. Other alternatives such as UV curing of the coating have been discovered and may be used.
A characteristic and limitation of powder coatings that is different from liquid paints is that when powder coatings of two different colors are blended together, the resultant finish typically has a speckled appearance rather than being uniform in color. For example, if a white powder coating is mixed with, or contaminated with, a black powder coating and then applied, the final coating will have a black and white speckled appearance, instead of having a uniform gray color finish.
This characteristic of powder coatings has important implications in the economics of powder coatings manufacture, as each powder composition must be separated from other powder compositions. However, it is difficult to quickly and inexpensively change from one powder coating composition color to another. Changing compositions requires complete separation of product from one batch of colored powder to the next at all stages of manufacturing. At the extruder, this typically requires a complete purge of the equipment, and then feeding the next composition until it stabilizes. This takes time and results in waste product. At the grinder, the entire grinding system must be cleaned. This typically requires major disassembly and cleaning of all grinding equipment and associated ductwork, and then reassembly, which is a time and labor intensive endeavor. Failure to fully clean the equipment will result in the second powder color composition having specks of the first color composition in it.
The cleaning process for switching from production of one color to another is particularly onerous in the production of small batches of special colors. For small batches, it is not uncommon for the cleaning process to consume more time and labor than was required to extrude and grind the batch.
A related problem occurs when a customer requests a custom color powder coating. If the color initially produced does not match the requested color, then it must be re-fed into the extruder with additional pigment or material. This requires the whole process, including clean up, to be repeated. Alternatively, the batch may be discarded and an entirely new batch run to obtain the proper color match.
For all of these reasons, it is difficult, time consuming, and expensive to produce small amounts of any particular powder coating color. It is greatly preferred to produce large amounts of the same color, with minimal changeovers. However, due to the overall advantages of powder coatings, there is a growing acceptance of powder coatings. This is turn has lead to a greater demand for a wider variety of colors for an increasing array of applications.